Electron transfer is activated by calmodulin in the flavin domain of human neuronal nitric oxide synthase

Conformational States and Fluctuations in Endothelial Nitric Oxide Synthase under Calmodulin Regulation

Mechanisms that regulate nitric oxide synthase enzymes (NOS) are of interest in biology and medicine. Although NOS catalysis relies on domain motions and is activated by calmodulin (CaM) binding, the relationships are unclear. We used single-molecule fluorescence resonance energy transfer (FRET) spectroscopy to elucidate the conformational states distribution and associated conformational fluctuation dynamics of the two NOS electron transfer domains in a FRET dye-labeled endothelial NOS reductase domain (eNOSr) and to understand how CaM affects the dynamics to regulate catalysis by shaping the spatial and temporal conformational behaviors of eNOSr. In addition, we developed and applied a new imaging approach capable of recording 3D FRET efficiency vs time images to characterize the impact on dynamic conformal states of the eNOSr enzyme by the binding of CaM, which identifies clearly that CaM binding generates an extra new open state of eNOSr, resolving more detailed NOS conformational states and their fluctuation dynamics. We identified a new output state that has an extra-open FAD-FMN conformation that is only populated in the CaM-bound eNOSr. This may reveal the critical role of CaM in triggering NOS activity as it gives conformational flexibility for eNOSr to assume the electron transfer output FMN-Heme state. Our results provide a dynamic link to recently reported EM static structure analyses and demonstrate a capable approach in probing and simultaneously analyzing all of the conformational states, their fluctuations, and the fluctuation dynamics for understanding the mechanism of NOS electron transfer, involving electron transfer amongst FAD, FMN, and Heme domains, during NO synthesis.

Neuronal Nitric Oxide Synthase (nNOS) in Neutrophils: An Insight

NO (nitric oxide) is an important regulator of neutrophil functions and has a key role in diverse pathophysiological conditions. NO production by nitric oxide synthases (NOS) is under tight control at transcriptional, translational, and post-translational levels including interactions with heterologous proteins owing to its potent chemical reactivity and high diffusibility; this limits toxicity to other cellular components and promotes signaling specificity. The protein-protein interactions govern the activity and spatial distribution of NOS isoform to regulatory proteins and to their intended targets. In comparison with the vast literature available for endothelial, macrophages, and neuronal cells, demonstrating neuronal NOS (nNOS) interaction with other proteins through the PDZ domain, neutrophil nNOS, however, remains unexplored. Neutrophil's key role in both physiological and pathological conditions necessitates the need for further studies in delineating the NOS mediated NO modulations in signaling pathways operational in them. nNOS has been linked to depression, schizophrenia, and Parkinson's disease, suggesting the importance of exploring nNOS/NO-mediated neutrophil physiology in relation to such neuronal disorders. The review thus presents the scenario of neutrophil nNOS from the genetics to the functional level, including protein-protein interactions governing its intracellular sequestration in diverse cell types, besides speculating possible regulation in neutrophils and also addressing their clinical implications.

Short-lived neutral FMN and FAD semiquinones are transient intermediates in cryo-reduced yeast NADPH-cytochrome P450 reductase

The electron configuration of flavin cofactors, FMN and FAD, is a critical factor governing the reactivity of NADPH-cytochrome P450 reductase (CPR). The current view of electron transfer by the mammalian CPR, based on equilibrium redox potentials of the flavin cofactors, is that the two electron-reduced FMN hydroquinone (FMNH2), rather than one electron-reduced FMN semiquinone, serves as electron donor to the terminal protein acceptors. However, kinetic and thermodynamic studies on the CPR species originated from different organisms have shown that redox potentials measured at distinct electron transfer steps differ from redox potentials determined by equilibrium titration. Collectively, previous observations suggest that the short-lived transient semiquinone species may carry electrons in diflavin reductases. In this work, we have investigated spectroscopic properties of the CPR-bound FAD and FMN reduced at 77 K by radiolytically-generated thermalized electrons. Using UV-vis spectroscopy, we demonstrated that upon cryo-reduction of oxidized yeast CPR (yCPR) containing an equimolar ratio of both FAD and FMN, or FAD alone, neutral semiquinones were trapped at 77 K. During annealing at the elevated temperatures, unstable short-lived neutral semiquinones relaxed to spectroscopically distinct air-stable neutral semiquinones. This transition was independent of pH within the 6.0-10.7 range. Our data on yeast CPR are in line with the previous observations of others that the flavin short-lived transient semiquinone intermediates may have a role in the electron transfer by CPR at physiological conditions.

Molecular mechanism of metabolic NAD(P)H-dependent electron-transfer systems: The role of redox cofactors

NAD(P)H-dependent electron-transfer (ET) systems require three functional components: a flavin-containing NAD(P)H-dehydrogenase, one-electron carrier and metal-containing redox center. In principle, these ET systems consist of one-, two- and three-components, and the electron flux from pyridine nucleotide cofactors, NADPH or NADH to final electron acceptor follows a linear pathway: NAD(P)H → flavin → one-electron carrier → metal containing redox center. In each step ET is primarily controlled by one- and two-electron midpoint reduction potentials of protein-bound redox cofactors in which the redox-linked conformational changes during the catalytic cycle are required for the domain-domain interactions. These interactions play an effective ET reactions in the multi-component ET systems. The microsomal and mitochondrial cytochrome P450 (cyt P450) ET systems, nitric oxide synthase (NOS) isozymes, cytochrome b₅ (cyt b₅) ET systems and methionine synthase (MS) ET system include a combination of multi-domain, and their organizations display similarities as well as differences in their components. However, these ET systems are sharing of a similar mechanism. More recent structural information obtained by X-ray and cryo-electron microscopy (cryo-EM) analysis provides more detail for the mechanisms associated with multi-domain ET systems. Therefore, this review summarizes the roles of redox cofactors in the metabolic ET systems on the basis of one-electron redox potentials. In final Section, evolutionary aspects of NAD(P)H-dependent multi-domain ET systems will be discussed.

The FNR modules contribute to control nitric oxide synthase catalysis revealed by chimera enzymes

The reductase domains of neuronal NOS, endothelial NOS and two constitutive nitric oxide synthase (cNOS) share higher sequence similarity (>60%). In order to evaluate the role of ferredoxin‑NADP+ reductase (FNR) module in adjusting NOS catalytic activities, chimeras were by interchanging the FNR‑like module between endothelial NOS and neuronal NOS in the present study. The assays of steady‑state enzymatic activities for cytochrome c and ferricyanide reduction, NO synthesis and NADPH oxidation were performed spectrophotometrically. The two NOS FNR modules transferred their ferricyanide reductase character to the chimera enzymes. Results showed that the FNR module was important in adjusting electrons flow through the reductase domain and out of the FMN module. Results indicated that the FNR module was critical in controlling the electron transfer capacities of the FMN module.

Phosphorylation Controls Endothelial Nitric-oxide Synthase by Regulating Its Conformational Dynamics

The activity of endothelial NO synthase (eNOS) is triggered by calmodulin (CaM) binding and is often further regulated by phosphorylation at several positions in the enzyme. Phosphorylation at Ser¹¹⁷⁹ occurs in response to diverse physiologic stimuli and increases the NO synthesis and cytochrome c reductase activities of eNOS, thereby enhancing its participation in biological signal cascades. Despite its importance, the mechanism by which Ser¹¹⁷⁹ phosphorylation increases eNOS activity is not understood. To address this, we used stopped-flow spectroscopy and computer modeling approaches to determine how the phosphomimetic mutation (S1179D) may impact electron flux through eNOS and the conformational behaviors of its reductase domain, both in the absence and presence of bound CaM. We found that S1179D substitution in CaM-free eNOS had multiple effects; it increased the rate of flavin reduction, altered the conformational equilibrium of the reductase domain, and increased the rate of its conformational transitions. We found these changes were equivalent in degree to those caused by CaM binding to wild-type eNOS, and the S1179D substitution together with CaM binding caused even greater changes in these parameters. The modeling indicated that the changes caused by the S1179D substitution, despite being restricted to the reductase domain, are sufficient to explain the stimulation of both the cytochrome c reductase and NO synthase activities of eNOS. This helps clarify how Ser¹¹⁷⁹ phosphorylation regulates eNOS and provides a foundation to compare its regulation by other phosphorylation events.

Single-molecule spectroscopy reveals how calmodulin activates NO synthase by controlling its conformational fluctuation dynamics

Mechanisms that regulate the nitric oxide synthase enzymes (NOS) are of interest in biology and medicine. Although NOS catalysis relies on domain motions, and is activated by calmodulin binding, the relationships are unclear. We used single-molecule fluorescence resonance energy transfer (FRET) spectroscopy to elucidate the conformational states distribution and associated conformational fluctuation dynamics of the two electron transfer domains in a FRET dye-labeled neuronal NOS reductase domain, and to understand how calmodulin affects the dynamics to regulate catalysis. We found that calmodulin alters NOS conformational behaviors in several ways: It changes the distance distribution between the NOS domains, shortens the lifetimes of the individual conformational states, and instills conformational discipline by greatly narrowing the distributions of the conformational states and fluctuation rates. This information was specifically obtainable only by single-molecule spectroscopic measurements, and reveals how calmodulin promotes catalysis by shaping the physical and temporal conformational behaviors of NOS.

Charge-pairing interactions control the conformational setpoint and motions of the FMN domain in neuronal nitric oxide synthase

The NOS (nitric oxide synthase; EC 1.14.13.39) enzymes contain a C-terminal flavoprotein domain [NOSred (reductase domain of NOS)] that binds FAD and FMN, and an N-terminal oxygenase domain that binds haem. Evidence suggests that the FMN-binding domain undergoes large conformational motions to shuttle electrons between the NADPH/FAD-binding domain [FNR (ferredoxin NADP-reductase)] and the oxygenase domain. Previously we have shown that three residues on the FMN domain (Glu762, Glu816 and Glu819) that make charge-pairing interactions with the FNR help to slow electron flux through nNOSred (neuronal NOSred). In the present study, we show that charge neutralization or reversal at each of these residues alters the setpoint [Keq(A)] of the NOSred conformational equilibrium to favour the open (FMN-deshielded) conformational state. Moreover, computer simulations of the kinetic traces of cytochrome c reduction by the mutants suggest that they have higher conformational transition rates (1.5-4-fold) and rates of interflavin electron transfer (1.5-2-fold) relative to wild-type nNOSred. We conclude that the three charge-pairing residues on the FMN domain govern electron flux through nNOSred by stabilizing its closed (FMN-shielded) conformational state and by retarding the rate of conformational switching between its open and closed conformations.

Dissecting the kinetics of the NADP(+)-FADH2 charge transfer complex and flavin semiquinones in neuronal nitric oxide synthase

Electron flow within the neuronal nitric oxide synthase reductase domain (nNOSrd) includes hydride transfer from NADPH to FAD followed by two one-electron transfer reactions from FAD to FMN. We have used stopped flow spectrometry to closely monitor these electron transfer steps for both the wild type and the ΔG810 mutant of nNOSrd using a protocol involving both global analyses of the photodiode array spectral scans and curve fittings of single wavelength kinetic traces. The charge transfer complex and interflavin electron transfer events recorded at 750nm and 600nm, respectively, show the kinetics in different time frames. All electron transfer events are slow enough at 4°C to enable measurements of rate constants even for the fast charge transfer event. To our knowledge this is the first time the rate constants for the charge transfer between NADP(+) and FADH2 have been determined for NOS. These procedures allow us to conclude that (1) binding of the second NADPH is necessary to drive the full reduction of FMN and; (2) charge transfer and the subsequent interflavin electron transfer have distinct spectral features that can be monitored separately with stopped flow spectroscopy. These studies also enable us to conclude that interflavin electron transfer reported at 600nm is not limiting in NOS catalysis.

NADPH-cytochrome P450 oxidoreductase: prototypic member of the diflavin reductase family

NADPH-cytochrome P450 oxidoreductase (CYPOR) and nitric oxide synthase (NOS), two members of the diflavin oxidoreductase family, are multi-domain enzymes containing distinct FAD and FMN domains connected by a flexible hinge. FAD accepts a hydride ion from NADPH, and reduced FAD donates electrons to FMN, which in turn transfers electrons to the heme center of cytochrome P450 or NOS oxygenase domain. Structural analysis of CYPOR, the prototype of this enzyme family, has revealed the exact nature of the domain arrangement and the role of residues involved in cofactor binding. Recent structural and biophysical studies of CYPOR have shown that the two flavin domains undergo large domain movements during catalysis. NOS isoforms contain additional regulatory elements within the reductase domain that control electron transfer through Ca(2+)-dependent calmodulin (CaM) binding. The recent crystal structure of an iNOS Ca(2+)/CaM-FMN construct, containing the FMN domain in complex with Ca(2+)/CaM, provided structural information on the linkage between the reductase and oxgenase domains of NOS, making it possible to model the holo iNOS structure. This review summarizes recent advances in our understanding of the dynamics of domain movements during CYPOR catalysis and the role of the NOS diflavin reductase domain in the regulation of NOS isozyme activities.

Control of electron transfer and catalysis in neuronal nitric-oxide synthase (nNOS) by a hinge connecting its FMN and FAD-NADPH domains

In nitric-oxide synthases (NOSs), two flexible hinges connect the FMN domain to the rest of the enzyme and may guide its interactions with partner domains for electron transfer and catalysis. We investigated the role of the FMN-FAD/NADPH hinge in rat neuronal NOS (nNOS) by constructing mutants that either shortened or lengthened this hinge by 2, 4, and 6 residues. Shortening the hinge progressively inhibited electron flux through the calmodulin (CaM)-free and CaM-bound nNOS to cytochrome c, whereas hinge lengthening relieved repression of electron flux in CaM-free nNOS and had no impact or slowed electron flux through CaM-bound nNOS to cytochrome c. How hinge length influenced heme reduction depended on whether enzyme flavins were pre-reduced with NADPH prior to triggering heme reduction. Without pre-reduction, changing the hinge length was deleterious; with pre-reduction, the hinge shortening was deleterious, and hinge lengthening increased heme reduction rates beyond wild type. Flavin fluorescence and stopped-flow kinetic studies on CaM-bound enzymes suggested hinge lengthening slowed the domain-domain interaction needed for FMN reduction. All hinge length changes lowered NO synthesis activity and increased uncoupled NADPH consumption. We conclude that several aspects of catalysis are sensitive to FMN-FAD/NADPH hinge length and that the native hinge allows a best compromise among the FMN domain interactions and associated electron transfer events to maximize NO synthesis and minimize uncoupled NADPH consumption.

Differential regulation of inducible and endothelial nitric oxide synthase by kinin B1 and B2 receptors

Kinins are vasoactive peptides that play important roles in cardiovascular homeostasis, pain and inflammation. After release from their precursor kininogens, kinins or their C-terminal des-Arg metabolites activate two distinct G protein-coupled receptors (GPCR), called B2 (B2R) or B1 (B1R). The B2R is expressed constitutively with a wide tissue distribution. In contrast, the B1R is not expressed under normal conditions but is upregulated by tissue insult or inflammatory mediators. The B2R is considered to mediate many of the acute effects of kinins while the B1R is more responsible for chronic responses in inflammation. Both receptors can couple to Galphai and Galphaq families of G proteins to release mediators such as nitric oxide (NO), arachidonic acid, prostaglandins, leukotrienes and endothelium-derived hyperpolarizing factor and can induce the release of other inflammatory agents. The focus of this review is on the different transduction events that take place upon B2R and B1R activation in human endothelial cells that leads to generation of NO via activation of different NOS isoforms. Importantly, B2R-mediated eNOS activation leads to a transient ( approximately 5min) output of NO in control endothelial cells whereas in cytokine-treated endothelial cells, B1R activation leads to very high and prolonged ( approximately 90min) NO production that is mediated by a novel signal transduction pathway leading to post-translational activation of iNOS.

Lys842 in neuronal nitric-oxide synthase enables the autoinhibitory insert to antagonize calmodulin binding, increase FMN shielding, and suppress interflavin electron transfer

Neuronal nitric-oxide synthase (nNOS) contains a unique autoinhibitory insert (AI) in its FMN subdomain that represses nNOS reductase activities and controls the calcium sensitivity of calmodulin (CaM) binding to nNOS. How the AI does this is unclear. A conserved charged residue (Lys(842)) lies within a putative CaM binding helix in the middle of the AI. We investigated its role by substituting residues that neutralize (Ala) or reverse (Glu) the charge at Lys(842). Compared with wild type nNOS, the mutant enzymes had greater cytochrome c reductase and NADPH oxidase activities in the CaM-free state, were able to bind CaM at lower calcium concentration, and had lower rates of heme reduction and NO synthesis in one case (K842A). Moreover, stopped-flow spectrophotometric experiments with the nNOS reductase domain indicate that the CaM-free mutants had faster flavin reduction kinetics and had less shielding of their FMN subdomains compared with wild type and no longer increased their level of FMN shielding in response to NADPH binding. Thus, Lys(842) is critical for the known functions of the AI and also enables two additional functions of the AI as newly identified here: suppression of electron transfer to FMN and control of the conformational equilibrium of the nNOS reductase domain. Its effect on the conformational equilibrium probably explains suppression of catalysis by the AI.

Regulation of interdomain interactions by calmodulin in inducible nitric-oxide synthase

Nitric-oxide synthases (NOSs) catalyze the conversion of l-arginine to nitric oxide and citrulline. There are three NOS isozymes, each with a different physiological role: neuronal NOS, endothelial NOS, and inducible NOS (iNOS). NOSs consist of an N-terminal oxygenase domain and a C-terminal reductase domain, linked by a calmodulin (CaM)-binding region. CaM is required for NO production, but unlike other NOS isozymes, iNOS binds CaM independently of the exogenous Ca(2+) concentration. We have co-expressed CaM and the FMN domain of human iNOS, which includes the CaM-binding region. The Ca(2+)-bound protein complex (CaCaMxFMN) forms an air-stable semiquinone when reduced with NADPH and reduces cytochrome c when reconstituted with the iNOS FAD/NADPH domain. We have solved the crystal structure of the CaCaMxFMN complex in four different conformations, each with a different relative orientation, between the FMN domain and the bound CaM. The CaM-binding region together with bound CaM forms a hinge, pivots on the conserved Arg(536), and regulates electron transfer from FAD to FMN and from FMN to heme by adjusting the relative orientation and distance among the three cofactors. In addition, the relative orientations of the N- and C-terminal lobes of CaM are also different among the four conformations, suggesting that the flexibility between the two halves of CaM also contributes to the fine tuning of the orientation/distance between the redox centers. The data demonstrate a possible mode for precise control of electron transfer by altering the distance and orientation of redox centers in a protein displaying domain movement.

Structural and mechanistic aspects of flavoproteins: electron transfer through the nitric oxide synthase flavoprotein domain

Nitric oxide synthases belong to a family of dual-flavin enzymes that transfer electrons from NAD(P)H to a variety of heme protein acceptors. During catalysis, their FMN subdomain plays a central role by acting as both an electron acceptor (receiving electrons from FAD) and an electron donor, and is thought to undergo large conformational movements and engage in two distinct protein-protein interactions in the process. This minireview summarizes what we know about the many factors regulating nitric oxide synthase flavoprotein domain function, primarily from the viewpoint of how they impact electron input/output and conformational behaviors of the FMN subdomain.

Regulation of FMN subdomain interactions and function in neuronal nitric oxide synthase

Nitric oxide synthases (NOS) are modular, calmodulin- (CaM-) dependent, flavoheme enzymes that catalyze oxidation of l-arginine to generate nitric oxide (NO) and citrulline. During catalysis, the FMN subdomain cycles between interaction with an NADPH-FAD subdomain to receive electrons and interaction with an oxygenase domain to deliver electrons to the NOS heme. This process can be described by a three-state, two-equilibrium model for the conformation of the FMN subdomain, in which it exists in two distinct bound states (FMN-shielded) and one common unbound state (FMN-deshielded). We studied how each partner subdomain, the FMN redox state, and CaM binding may regulate the conformational equilibria of the FMN module in rat neuronal NOS (nNOS). We utilized four nNOS protein constructs of different subdomain composition, including the isolated FMN subdomain, and determined changes in the conformational state by measuring the degree of FMN shielding by fluorescence, electron paramagnetic resonance, or stopped-flow spectroscopic techniques. Our results suggest the following: (i) The NADPH-FAD subdomain has a far greater capacity to interact with the FMN subdomain than does the oxygenase domain. (ii) CaM binding has no direct effects on the FMN subdomain. (iii) CaM destabilizes interaction of the FMN subdomain with the NADPH-FAD subdomain but does not measurably increase its interaction with the oxygenase domain. Our results imply that a different set point and CaM regulation exists for either conformational equilibrium of the FMN subdomain. This helps to explain the unique electron transfer and catalytic behaviors of nNOS, relative to other dual-flavin enzymes.

Neuronal nitric oxide synthase: structure, subcellular localization, regulation, and clinical implications

Nitric oxide (NO), a free gaseous signaling molecule, is involved in the regulation of the cardiovascular, nervous and immune system. The neurotransmitter function of nitric oxide is dependent on dynamic regulation of its biosynthetic enzyme, nitric oxide synthase (NOS). There are three types of NOS, neuronal nitric oxide synthase (nNOS), endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS). Of the three NOS, we focus on nNOS in the present review. Brain nNOS exists in particulate and soluble forms and the differential subcellular localization of nNOS may contribute to its diverse functions. Proteins bearing PDZ domains can interact directly with the PDZ domain of nNOS, influencing the subcellular distribution and/or activity of the enzyme. During the past several years, an increasing number of reports have demonstrated the importance of nNOS in a variety of synaptic signaling events. nNOS has been implicated in modulating physiological functions such as learning, memory, and neurogenesis, as well as being involved in a number of human diseases. In this review we concentrate on recent findings regarding the structural features, subcellular localization and factors regulating nNOS function. In particular, we conclude with a section discussing the role of nNOS in a wide range of physiological and pathological conditions.

Importance of the domain-domain interface to the catalytic action of the NO synthase reductase domain

Calmodulin (CaM) activates NO synthase (NOS) by binding to a 20 amino acid interdomain hinge in the presence of Ca (2+), inducing electrons to be transferred from the FAD to the heme of the enzyme via a mobile FMN domain. The activation process is influenced by a number of structural features, including an autoinhibitory loop, the C-terminal tail of the enzyme, and a number of phosphorylation sites. Crystallographic and other recent experimental data imply that the regulatory elements lie within the interface between the FAD- and FMN-binding domains, restricting the movement of the two cofactors with respect to each other. Arg1229 of rat neuronal NOS is a conserved residue in the FAD domain that forms one of only two electrostatic contacts between the domains. Mutation of this residue to Glu reverses its charge and is expected to induce an interdomain repulsion, allowing the importance of the interface and domain-domain motion to be probed. The charge-reversal mutation R1229E has three dramatic effects on catalysis: (i) hydride transfer from NADPH to FAD is activated in the CaM-free enzyme, (ii) FAD to FMN electron transfer is inhibited in both forms, and (iii) electron transfer from FMN to the surrogate acceptor cytochrome c is activated in the CaM-free enzyme. As a result, during steady-state turnover with cytochrome c, calmodulin now deactivates the enzyme and causes cytochrome c-dependent inhibition. Evidently, domain-domain separation is large enough in the mutant to accommodate another protein between the cofactors. The effects of this single charge reversal on three distinct catalytic events illustrate how each is differentially dependent on the enzyme conformation and support a model for catalytic motion in which steps i, ii, and iii occur in the hinged open, closed, and open states, respectively. This model is also likely to apply to related enzymes such as cytochrome P450 reductase.

Differences in a conformational equilibrium distinguish catalysis by the endothelial and neuronal nitric-oxide synthase flavoproteins

Nitric oxide (NO) is a physiological mediator synthesized by NO synthases (NOS). Despite their structural similarity, endothelial NOS (eNOS) has a 6-fold lower NO synthesis activity and 6-16-fold lower cytochrome c reductase activity than neuronal NOS (nNOS), implying significantly different electron transfer capacities. We utilized purified reductase domain constructs of either enzyme (bovine eNOSr and rat nNOSr) to investigate the following three mechanisms that may control their electron transfer: (i) the set point and control of a two-state conformational equilibrium of their FMN subdomains; (ii) the flavin midpoint reduction potentials; and (iii) the kinetics of NOSr-NADP+ interactions. Although eNOSr and nNOSr differed in their NADP(H) interaction and flavin thermodynamics, the differences were minor and unlikely to explain their distinct electron transfer activities. In contrast, calmodulin (CaM)-free eNOSr favored the FMN-shielded (electron-accepting) conformation over the FMN-deshielded (electron-donating) conformation to a much greater extent than did CaM-free nNOSr when the bound FMN cofactor was poised in each of its three possible oxidation states. NADPH binding only stabilized the FMN-shielded conformation of nNOSr, whereas CaM shifted both enzymes toward the FMN-deshielded conformation. Analysis of cytochrome c reduction rates measured within the first catalytic turnover revealed that the rate of conformational change to the FMN-deshielded state differed between eNOSr and nNOSr and was rate-limiting for either CaM-free enzyme. We conclude that the set point and regulation of the FMN conformational equilibrium differ markedly in eNOSr and nNOSr and can explain the lower electron transfer activity of eNOSr.

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

Mechanistic studies on the intramolecular one-electron transfer between the two flavins in the human endothelial NOS reductase domain

The object of this study was to clarify the mechanism of electron transfer in the human endothelial nitric oxide synthase (eNOS) reductase domain using recombinant eNOS reductase domains; the FAD/NADPH domain containing FAD- and NADPH-binding sites and the FAD/FMN domain containing FAD/NADPH-, FMN-, and a calmodulin-binding sites. In the presence of molecular oxygen or menadione, the reduced FAD/NADPH domain is oxidized via the neutral (blue) semiquinone (FADH(*)), which has a characteristic absorption peak at 520 nm. The FAD/NADPH and FAD/FMN domains have high activity for ferricyanide, but the FAD/FMN domain has low activity for cytochrome c. In the presence or absence of calcium/calmodulin (Ca(2+)/CaM), reduction of the oxidized flavins (FAD-FMN) and air-stable semiquinone (FAD-FMNH(*)) with NADPH occurred in at least two phases in the absorbance change at 457nm. In the presence of Ca(2+)/CaM, the reduction rate of both phases was significantly increased. In contrast, an absorbance change at 596nm gradually increased in two phases, but the rate of the fast phase was decreased by approximately 50% of that in the presence of Ca(2+)/CaM. The air-stable semiquinone form was rapidly reduced by NADPH, but a significant absorbance change at 520 nm was not observed. These findings indicate that the conversion of FADH(2)-FMNH(*) to FADH(*)-FMNH(2) is unfavorable. Reduction of the FAD moiety is activated by CaM, but the formation rate of the active intermediate, FADH(*)-FMNH(2) is extremely low. These events could cause a lowering of enzyme activity in the catalytic cycle.

Intraprotein electron transfer in a two-domain construct of neuronal nitric oxide synthase: the output state in nitric oxide formation

Intersubunit intraprotein electron transfer (IET) from flavin mononucleotide (FMN) to heme is essential in nitric oxide (NO) synthesis by NO synthase (NOS). Previous crystal structures and functional studies primarily concerned an enzyme conformation, which serves as the input state for reduction of FMN by electrons from NADPH and flavin adenine dinucleotide (FAD) in the reductase domain. To favor the formation of the output state for the subsequent IET from FMN to heme in the oxygenase domain, a novel truncated two-domain oxyFMN construct of rat neuronal NOS (nNOS), in which only the FMN and heme domains were present, was designed and expressed. The kinetics of IET between the FMN and heme domains in the nNOS oxyFMN construct in the presence and absence of added calmodulin (CaM) were directly determined using laser flash photolysis of CO dissociation in comparative studies on partially reduced oxyFMN and single-domain heme oxygenase constructs. The IET rate constant in the presence of CaM (262 s(-)(1)) was increased approximately 10-fold compared to that in the absence of CaM (22 s(-)(1)). The effect of CaM on interdomain interactions was further evidenced by electron paramagnetic resonance (EPR) spectra. This work provides the first direct evidence of the CaM control of electron transfer (ET) between FMN and heme domains through facilitation of the FMN/heme interactions in the output state. Therefore, CaM controls IET between heme and FMN domains by a conformational gated mechanism. This is essential in coupling ET in the reductase domain in NOS with NO synthesis in the oxygenase domain.

Direct measurement by laser flash photolysis of intramolecular electron transfer in a two-domain construct of murine inducible nitric oxide synthase

Intersubunit intramolecular electron transfer (IET) from FMN to heme is essential in the delivery of electrons required for O2 activation in the heme domain and the subsequent nitric oxide (NO) synthesis by NO synthase (NOS). Previous crystal structures and functional studies primarily concerned an enzyme conformation that serves as the input state for reduction of FMN by electrons from NADPH and FAD in the reductase domain. To favor formation of the output state for the subsequent IET from FMN to heme in the oxygenase domain, a novel truncated two-domain oxyFMN construct murine inducible nitric oxide synthase (iNOS), in which only the FMN and heme domains were present, was designed and expressed. The kinetics of the IET between the FMN and heme domains in this construct was directly determined using laser flash photolysis of CO dissociation in comparative studies on partially reduced oxyFMN and single domain heme oxygenase constructs.

Structure and function of NADPH-cytochrome P450 reductase and nitric oxide synthase reductase domain

NADPH-cytochrome P450 reductase (CPR) and the nitric oxide synthase (NOS) reductase domains are members of the FAD-FMN family of proteins. The FAD accepts two reducing equivalents from NADPH (dehydrogenase flavin) and FMN acts as a one-electron carrier (flavodoxin-type flavin) for the transfer from NADPH to the heme protein, in which the FMNH*/FMNH2 couple donates electrons to cytochrome P450 at constant oxidation-reduction potential. Although the interflavin electron transfer between FAD and FMN is not strictly regulated in CPR, electron transfer is activated in neuronal NOS reductase domain upon binding calmodulin (CaM), in which the CaM-bound activated form can function by a similar mechanism to that of CPR. The oxygenated form and spin state of substrate-bound cytochrome P450 in perfused rat liver are also discussed in terms of stepwise one-electron transfer from CPR. This review provides a historical perspective of the microsomal mixed-function oxidases including CPR and P450. In addition, a new model for the redox-linked conformational changes during the catalytic cycle for both CPR and NOS reductase domain is also discussed.

Interflavin one-electron transfer in the inducible nitric oxide synthase reductase domain and NADPH-cytochrome P450 reductase

In this study, we have analyzed interflavin electron transfer reactions from FAD to FMN in both the full-length inducible nitric oxide synthase (iNOS) and its reductase domain. Comparison is made with the interflavin electron transfer in NADPH-cytochrome P450 reductase (CPR). For the analysis of interflavin electron transfer and the flavin intermediates observed during catalysis we have used menadione (MD), which can accept an electron from both the FAD and FMN sites of the enzyme. A characteristic absorption peak at 630 and 520 nm can identify each FAD and FMN semiquinone species, which is derived from CPR and iNOS, respectively. The charge transfer complexes of FAD with NADP+ or NADPH were monitored at 750 nm. In the presence of MD, the air-stable neutral (blue) semiquinone form (FAD-FMNH*) was observed as a major intermediate during the catalytic cycle in both the iNOS reductase domain and full-length enzyme, and its formation occurred without any lag phase indicating rapid interflavin electron transfer following the reduction of FAD by NADPH. These data also strongly suggest that the low level reactivity of a neutral (blue) FMN semiquinone radical with electron acceptors enables one-electron transfer in the catalytic cycle of both the FAD-FMN pairs in CPR and iNOS. On the basis of these data, we propose a common model for the catalytic cycle of both CaM-bound iNOS reductase domain and CPR.

C-terminal tail residue Arg1400 enables NADPH to regulate electron transfer in neuronal nitric-oxide synthase

The neuronal nitric-oxide synthase (nNOS) flavoprotein domain (nNOSr) contains regulatory elements that repress its electron flux in the absence of bound calmodulin (CaM). The repression also requires bound NADP(H), but the mechanism is unclear. The crystal structure of a CaM-free nNOSr revealed an ionic interaction between Arg(1400) in the C-terminal tail regulatory element and the 2'-phosphate group of bound NADP(H). We tested the role of this interaction by substituting Ser and Glu for Arg(1400) in nNOSr and in the full-length nNOS enzyme. The CaM-free nNOSr mutants had cytochrome c reductase activities that were less repressed than in wild-type, and this effect could be mimicked in wild-type by using NADH instead of NADPH. The nNOSr mutants also had faster flavin reduction rates, greater apparent K(m) for NADPH, and greater rates of flavin auto-oxidation. Single-turnover cytochrome c reduction data linked these properties to an inability of NADP(H) to cause shielding of the FMN module in the CaM-free nNOSr mutants. The full-length nNOS mutants had no NO synthesis in the CaM-free state and had lower steady-state NO synthesis activities in the CaM-bound state compared with wild-type. However, the mutants had faster rates of ferric heme reduction and ferrous heme-NO complex formation. Slowing down heme reduction in R1400E nNOS with CaM analogues brought its NO synthesis activity back up to normal level. Our studies indicate that the Arg(1400)-2'-phosphate interaction is a means by which bound NADP(H) represses electron transfer into and out of CaM-free nNOSr. This interaction enables the C-terminal tail to regulate a conformational equilibrium of the FMN module that controls its electron transfer reactions in both the CaM-free and CaM-bound forms of nNOS.

Inhibitory effect of organotin compounds on rat neuronal nitric oxide synthase through interaction with calmodulin

Organotin compounds, triphenyltin (TPT), tributyltin, dibutyltin, and monobutyltin (MBT), showed potent inhibitory effects on both L-arginine oxidation to nitric oxide and L-citrulline, and cytochrome c reduction catalyzed by recombinant rat neuronal nitric oxide synthase (nNOS). The two inhibitory effects were almost parallel. MBT and TPT showed the highest inhibitory effects, followed by tributyltin and dibutyltin; TPT and MBT showed inhibition constant (IC(50)) values of around 10microM. Cytochrome c reduction activity was markedly decreased by removal of calmodulin (CaM) from the complete mixture, and the decrease was similar to the extent of inhibition by TPT and MBT. The inhibitory effect of MBT on the cytochrome c reducing activity was rapidly attenuated upon dilution of the inhibitor, and addition of a high concentration of CaM reactivated the cytochrome c reduction activity inhibited by MBT. However, other cofactors such as FAD, FMN or tetrahydrobiopterin had no such ability. The inhibitory effect of organotin compounds (100microM) on L-arginine oxidation of nNOS almost vanished when the amount of CaM was sufficiently increased (150-300microM). It was confirmed by CaM-agarose column chromatography that the dissociation of nNOS-CaM complex was induced by organotin compounds. These results indicate that organotin compounds disturb the interaction between CaM and nNOS, thereby inhibiting electron transfer from the reductase domain to cytochrome c and the oxygenase domain.

Human neuronal nitric oxide synthase can catalyze one-electron reduction of adriamycin: role of flavin domain

We have analyzed the mechanism of one-electron reduction of adriamycin (Adr) using recombinant full-length human neuronal nitric-oxide synthase and its flavin domains. Both enzymes catalyzed aerobic NADPH oxidation in the presence of Adr. Calcium/calmodulin (Ca(2+)/CaM) stimulated the NADPH oxidation of Adr. In the presence or absence of Ca(2+)/CaM, the flavin semiquinone radical species were major intermediates observed during the oxidation of the reduced enzyme by Adr. The FAD-NADPH binding domain did not significantly catalyze the reduction of Adr. Neither the FAD semiquinone (FADH*) nor the air-stable semiquinone (FAD-FMNH*) reacted rapidly with Adr. These data indicate that the fully reduced species of FMN (FMNH(2)) donates one electron to Adr, and that the rate of Adr reduction is stimulated by a rapid electron exchange between the two flavins in the presence of Ca(2+)/CaM. Based on these findings, we propose a role for the FAD-FMN pair in the one-electron reduction of Adr.

The FAD-shielding residue Phe1395 regulates neuronal nitric-oxide synthase catalysis by controlling NADP+ affinity and a conformational equilibrium within the flavoprotein domain

Phe(1395) stacks parallel to the FAD isoalloxazine ring in neuronal nitric-oxide synthase (nNOS) and is representative of conserved aromatic amino acids found in structurally related flavoproteins. This laboratory previously showed that Phe(1395) was required to obtain the electron transfer properties and calmodulin (CaM) response normally observed in wild-type nNOS. Here we characterized the F1395S mutant of the nNOS flavoprotein domain (nNOSr) regarding its physical properties, NADP(+) binding characteristics, flavin reduction kinetics, steady-state and pre-steady-state cytochrome c reduction kinetics, and ability to shield its FMN cofactor in response to CaM or NADP(H) binding. F1395S nNOSr bound NADP(+) with 65% more of the nicotinamide ring in a productive conformation with FAD for hydride transfer and had an 8-fold slower rate of NADP(+) dissociation. CaM stimulated the rates of NADPH-dependent flavin reduction in wild-type nNOSr but not in the F1395S mutant, which had flavin reduction kinetics similar to those of CaM-free wild-type nNOSr. CaM-free F1395S nNOSr lacked repression of cytochrome c reductase activity that is typically observed in nNOSr. The combined results from pre-steady-state and EPR experiments revealed that this was associated with a lesser degree of FMN shielding in the NADP(+)-bound state as compared with wild type. We conclude that Phe(1395) regulates nNOSr catalysis in two ways. It facilitates NADP(+) release to prevent this step from being rate-limiting, and it enables NADP(H) to properly regulate a conformational equilibrium involving the FMN subdomain that controls reactivity of the FMN cofactor in electron transfer.

A conserved aspartate (Asp-1393) regulates NADPH reduction of neuronal nitric-oxide synthase: implications for catalysis

Nitric-oxide synthases (NOSs) are flavo-heme enzymes whose electron transfer reactions are controlled by calmodulin (CaM). The NOS flavoprotein domain includes a ferredoxin-NADP(+) reductase (FNR)-like module that contains NADPH- and FAD-binding sites. FNR-like modules in related flavoproteins have three conserved residues that regulate electron transfer between bound NAD(P)H and FAD. To investigate the function of one of these residues in neuronal NOS (nNOS), we generated and characterized mutants that had Val, Glu, or Asn substituted for the conserved Asp-1393. All three mutants exhibited normal composition, spectral properties, and binding of cofactors, substrates, and CaM. All had slower NADPH-dependent cytochrome c and ferricyanide reductase activities, which were associated with proportionally slower rates of NADPH-dependent flavin reduction in the CaM-free and CaM-bound states. Rates of NO synthesis were also proportionally slower in the mutants and were associated with slower rates of CaM-dependent ferric heme reduction. However, a D1393V mutant whose flavins had been prereduced with NADPH had a normal rate of heme reduction. This indicated that the kinetic defect was restricted to flavin reduction step(s) in the mutants and suggested that this limited their catalytic activities. Together, our results show the following. 1) The presence and positioning of the Asp-1393 carboxylate side chain are critical to enable NADPH-dependent reduction of the nNOS flavoprotein. 2) Control of flavin reduction is important because it ensures that the rate of heme reduction is sufficiently fast to enable NO synthesis by nNOS.

Subsecond separation of cellular flavin coenzymes by microchip capillary electrophoresis with laser-induced fluorescence detection

In this article, it was demonstrated that a subsecond separation of cellular metabolites such as riboflavin (RF), flavin mononucleotide (FMN), and flavin-adenine dinucleotide (FAD) was achieved using microchip capillary electrophoresis with laser-induced fluorescence detection. The influences of crucial parameters that governed analysis time (e.g., channel length and electric field for separation) and separation resolution (e.g., sample size) were investigated, both in theoretical aspects and experimental practice. Quantitative analyses were performed that exhibited linear dynamic range of two orders of magnitude, with calculated detection limits of 34, 201, and 127 nM for RF, FAD, and FMN, respectively. To test the validity of the method, it was successfully applied to characterize several recombinant flavin-binding domains in a human neuronal nitric oxide synthase.

Mechanistic studies on the intramolecular one-electron transfer between the two flavins in the human neuronal nitric-oxide synthase and inducible nitric-oxide synthase flavin domains

Neuronal nitric-oxide synthase (nNOS) differs from inducible NOS (iNOS) in both its dependence on the intracellular Ca2+ concentration and the production rate of NO. To investigate what difference(s) exist between the two NOS flavin domains at the electron transfer level, we isolated the recombinant human NOS flavin domains, which were co-expressed with human calmodulin (CaM). The flavin semiquinones, FADH* and FMNH*, in both NOSs participate in the regulation of one-electron transfer within the flavin domain. Each semiquinone can be identified by a characteristic absorption peak at 520 nm (Guan, Z.-W., and Iyanagi, T. (2003) Arch. Biochem. Biophys. 412, 65-76). NADPH reduction of the FAD and FMN redox centers by the CaM-bound flavin domains was studied by stopped-flow and rapid scan spectrometry. Reduction of the air-stable semiquinone (FAD-FMNH*) of both domains with NADPH showed that the extent of conversion of FADH2/FMNH* to FADH*/FMNH2 in the iNOS flavin domain was greater than that of the nNOS flavin domain. The reduction of both oxidized domains (FAD-FMN) with NADPH resulted in the initial formation of a small amount of disemiquinone, which then decayed. The rate of intramolecular electron transfer between the two flavins in the iNOS flavin domain was faster than that of the nNOS flavin domain. In addition, the formation of a mixture of the two- and four-electron-reduced states in the presence of excess NADPH was different for the two NOS flavin domains. The data indicate a more favorable formation of the active intermediate FMNH2 in the iNOS flavin domain.